A former Australian prime minister who was a vocal leader of the national campaign against gay marriage has attended the same-sex wedding of his sister.

Tony Abbott, who was replaced as prime minister in 2015 in an internal government power struggle, joined his sister Christine Forster when she married her partner Virginia Flitcroft in a civil ceremony overlooking the Sydney Opera House.

Mr Abbot welcomed his sister-in-law into his family, saying: “Great family occasion. Very happy for Chris and Virginia. I’m looking forward to having a new sister-in-law.”

A former Roman Catholic seminarian, Mr Abbott angered gay rights advocates and many in his conservative government by deciding in 2015 to put the gay marriage issue to an extraordinary national vote.

His explanation was to spare his government colleagues an ugly and divisive debate to resolve the issue.

Critics suspected he was betting that Australia’s recent history of resisting change in referendums would maintain the ban on same-sex marriage. But 62% of voting Australians who took part in a postal survey last year opted for change. Parliament overwhelmingly supported legislation lifting the ban in December.